1. Field of the Invention
This invention pertains to cutting apparatus, and more particularly to apparatus for cutting and stripping insulation from insulated electrical conductors. 2. Description of the Prior Art
Industry utilizes two basic types of cutting blades for high production stripping of insulation from insulated electrical conductors, the die type and the V type. In the die type design, FIGS. 1a and 1b, a pair of knife blades 1 form two halves of a counterbored hole 2 and a smaller through hole 4. The two blades 1 are closed over the insulation 3 of a specifically sized insulated conductor 5 at the desired location thereon. The die type design possesses the advantages of closely conforming to the configuration of the conductor 7 around the full periphery thereof and of having a depth of cut controlled by the contacting of leading edges 9 to prevent nicking and scoring the conductor. Nicking a conductor is undesirable because nicks decrease the electrical current carrying capacity, and they produce stress concentrations which decrease the tensile strength of the conductor. However, die type blades suffer from three disadvantages. The first is that a pair of die blades is designed to be used with only one size of insulated conductor. Secondly, the squeezing of the insulation 3 between the conductor 7 and the blades 1 may prevent the blades from fully closing onto the conductor. The insulation is then not completely cut through to the conductor. Rather, some of the insulation is squeezed in the area 11 between the leading edges 9 and the conductor. As a result, the uncut insulation must be torn from the parent insulation. The third disadvantage is that die type blades have only a low tolerance for accommodating off-center insulated conductors transported between the open blades. In many applications, special wire guides in the form of mechanical fingers are required to ensure the insulated conductor is on the center line of the closed blades.
Referring to FIGS. 2a and 2b, a pair of conventional V type blades 13 are shown. Each blade 13 is manufactured with a sharp edged cutting radius 15 at the bottom of the V 17 for slicing through the insulation 3. The cutting radius 15 is ideally equal to one-half of the outer diameter of the conductor to be stripped, but a single pair of V type blades can cut and strip several sizes of insulated conductor. Another advantage of the V type blade design is that the sharp knife edges slice through the insulation to the conductor rather than squeezing the insulation. On the other hand, conventional V type designs lack positive stroke control; therefore the conductor 7 is prone to be nicked and scored when stripped.
Conventional V type blades are capable of gathering off-set insulated conductors 5 to the cutting edge 15 because of the V-shaped path 17 in each blade. The narrow end of the V path 17 is tangent to the cutting radius 15. The edges 19 which define the path 17 diverge from the radius cutting edge 15 at a blade angle D. It is apparent that a large angle D is desirable so as to gather greatly off-set insulated conductors. However, because of the angle D, the cutting radii 15 do not form a full circle around the conductor 7 when the blades 13 are closed, as shown in FIG. 3. Instead, a small amount of insulation thickness t and corresponding tear area a remain uncut. The insulation area a must be torn from the parent insulation during the stripping operation. The physical characteristics of some insulation materials make them difficult to tear, so it is desirable to minimize thickness t and area a.
It will be appreciated that insulation tear thickness t and area a would be eliminated if angle D were zero degrees. That ideal is not attainable in industrial practice because of insulated conductor size variations and inherent limitations on the cutting and stripping machines employing the stripping blades. Further, if the blade edges 17 were parallel, the blades would not tolerate off-center insulated conductors any better than die type blades, and they would be able to process only a single size insulated conductor. Thus, conventional stripping blades must compromise between a wide V for gathering ability and a narrow V for minimizing the amount of insulation remaining uncut.
The mathematical relationship between the roundness of the blade cutting edge and the blade angle D is expressed in terms of the percent circumferential contact between the blade cutting radius 15 and the conductor 7 periphery. The percent circumferential contact is defined as ##EQU1## The percent circumferential contact is independent of the size of the radius 15 and is affected only by blade angle D. Experimentation has shown that C should be at least 75% for efficient cutting and stripping. However, industrial standards for percent circumferential contact range as low as about 64% for a conductor diameter of 0.03125 inches and a blade angle D of about 60.degree., as shown by Table 1.
TABLE 1 ______________________________________ CON- BLADE GATH- CIRCUM- DUCTOR CUTTING ERING FERENTIAL DIAMETER EDGE RADIUS ANGLE D % CONTACT C ______________________________________ 0.03125" 0.01562" 60-64.degree. 64.4-66.7% 0.06250 0.03125 58-62 65.6-67.8 0.09375 0.04687 54-60 66.7-70.0 0.12500 0.06250 48-52 71.1-73.3 0.15625 0.07812 44-50 72.2-75.6 0.18750 0.09375 38-42 76.7-78.9 0.25000 0.12500 30-34 81.1-83.3 ______________________________________